I just want to share with the forum what I just found out:
I created a backup job with 3 VMs a few months ago. I named it "Backup 3 Servers" and it worked without problems for many weeks
Then I edited the job: I added a 4th VM and renamed the job to "Backup 4 Servers". The job was running without problems since then until last week.
I got errors during HealthCheck (Hint: Check Backup window!) and opened a support case.
Support asked me to check the backup files with validator. To do so I had to enter the following command: c:\Program Files\Veeam\Backup and Replication\Backup\Veeam.Backup.Validator.exe /backup:"Backup 4 Servers"
This resulted in Cannot find backup with name "Backup 4 Servers".
After some research I found out that renaming a job doesn't rename the backup on disk! So the folder on the repository still has the original name and therefore the correct command was: c:\Program Files\Veeam\Backup and Replication\Backup\Veeam.Backup.Validator.exe /backup:"Backup 3 Servers"
Now validator was running and reported no incomplete or failed VMs
Lesson learned: Renaming a job doesn't rename the backup!
Loving (and fighting with) Veeam for over a decade!